Enhancing bondability with coated copper bonding wire

There is growing interest in Cu wire bonding for LSI interconnection due to the cost savings and better electrical and mechanical properties. However, the scope of use for Cu bonding wires is generally severely limited compared to Au wires; e.g. for wire oxidation, lower bondability, forming gas of N2 + 5%H2, and lower reliability. It is difficult for conventional bare Cu wires to achieve the target of LSI application.A coated Cu wire (EX1) has thus been developed. It is a Pd-coated Cu wire and has many advantages compared to bare Cu wires. Its stitch strength was far superior under fresh conditions and remained constant without any deterioration after being stored in air for a prolonged period. EX1 had a lifetime of over 90 days in air, as compared to just 7 days for bare Cu wire. Spherical balls were formed with pure N2 (hydrogen-free), whereas the bare Cu produced off-center balls. Finally, the cost-effective and secure gas, pure N2 was only available for EX1. The excellent performance of the EX1 coated Cu wire is comparable to that of Au wires, making it suitable for LSI packaging.